Electronic flash apparatus

ABSTRACT

An electronic flash apparatus for automatically terminating the flash produced by the flash tube of the device after a condition where a predetermined total quantity of light has been received from the flashed object by a light responsive portion of the device, in which a series connection of a commutating capacitor and an auxiliary switching element is connected in parallel with a series connection of the flash tube and a main switching element, so that the auxiliary switching element is arranged in the same polarity as the main switching element. The commutating capacitor is charged in the reverse voltage to the charged voltage of the main capacitor. The flashing current of the flash tube is terminated by rendering the auxiliary switching element conductive in response to an ignition pulse generated at the above condition, while no commutating current is flowed in the flash tube. Another auxiliary switching element may be inserted in the former series connection so as to form a series connection of a switching element, the commutating capacitor and the auxiliary switching element.

United States Patent [191 Shimamura et al.

[111 3,849,703 Nov. 19, 1974 ELECTRONIC FLASH APPARATUS [75] Inventors:Akira Shimamura, Kawagoe; Kunio Matsuzaki, Hanno, both of Japan [73]Assignee: Shindengen Kogyo Kabushiki Kaisha (a/k/a Shindengen ElectricMfg. Co., Ltd., Tokyo-to, Japan [22] Filed: June 7, 1972 [21] Appl. No.:260,499

[30] Foreign Application Priority Data July 6, 1971 Japan 46-49284 June9, 1971 Japan .r 46-47987 July 20, 1971 Japan 46-63478 Aug. 31, 1971Japan 46-78492 Aug. 31, 1971 Japan 46-78493 Novjl, 1971 Japan 46-101841Mar. 18, 1972 Japan 47-32431 [52] 11.8. Cl. 315/241 P, 315/151, 315/157,315/159 [51] Int. Cl. l-l05b 41/40 [58] Field of Search 315/241 P, 151,156, 157, 315/159, 241 R [56] References Cited UNITED STATES PATENTS3,683,233 8/1972 Heintze 315/241 P 3,696,268 10/1972 Exner 315/1513,714,443 1/1973 Ogawa 315/241 P 3,740,610 6/1973 Roncke 315/241 PPrimary Examiner-Ronald L. Wibert Assistant Examiner-Richard A.Rosenberger Attorney, Agent, or Firm-Robert E. Burns; Emmanuel J.Lobato; Bruce L. Adams 57 ABSTRACT An electronic flash apparatus forautomatically terminating the flash produced by the flash tube of thedevice after a condition where a predetermined total quantity of lighthas been received from the flashed object by a light responsive portionof the device, in which a series connection of a commutating capacitorand an auxiliary switching element is connected in parallel with aseries connection of the flash tube and a main switching element, sothat the auxiliary switching element is arranged in the same polarity asthe main switching element. The commutating capacitor is charged in thereverse voltage to the charged voltage of the main capacitor. Theflashing current of the flash tube is terminated by rendering theauxiliary switching element conductive in response to an ignition pulsegenerated at the above condition, while no commutating current is flowedin the flash tube. Another auxiliary switching element may be insertedin the former series connection so as to form a series connection of aswitching element, the commutating capacitor and the auxiliary switchingelement.

13 Claims, 15 Drawing Figures PATENTELNUVIQIBH v 3,849'703 snssn gFIRING- CIR cu/ r PATENTE rsuv 1 91914 3.849.703 SHEEI 20F FIRINGCIRCUIT ELECTRONIC FLASH APPARATUS This invention relates to electronicflash apparatus using a flash tube and, more particularly, to electronicflash apparatus with automatic light termination.

Electronic photographic flash devices are known in the art in which theflash produced by the flash tube of the device is automaticallyterminated after a predetermined total quantity of light has beenreceived from the flashed object by a light responsive portion of thedevice. A current by-pass type and a current cut-off type have beenproposed in the art for the electronic flash devices for the type. Inthe device of the current bypass type, a by-pass tube having aconductive resistance smaller than the conductive resistance of theflash tube is connected in parallel, so that energy charged in the maincapacitor is consumed through the bypass tube when a predetermined totalquantity of liqht is projected to the flash object. Since the chargedenergy is completely discharged, a long time is necessary for chargingthe main capacitor in the device of the current by-pass type.Accordingly, successive flashes cannot be performed in the device of thecurrent by-pass type, .while power consumption in a battery'used forcharging the main capacitor is large. In the device of the currentcut-off type, a series connection of the flash tube and a firstthyrister is connected in parallel with the main capacitor, while aseries connection of a commutating capacitor and a second thyrister isconnected in parallel with the first thyrister so as to form a flip-flopcircuit. The flashing current of the flash tube is cut off by renderingthe second thyrister conductive by use of charged energy of thecommutating capacitor after a predetermined total quantity of light hasbeen received from the flashed object. However, a peak current having apeak value twice the normal flashing current is flowed in the flash tubeat the commutation time. Since the flash tube is strongly flashed by thepeak current, the flashed object is over-flashed in each exposure.

An object of this invention is to provide an electronic flash apparatuscapable of performing successive flashes in low power consumptionwithout over or least exposure.

In accordance with the principle of this invention, a firstseries-connection of a reactor, the flash tube and a main switchingelement is connected in parallel with a main capacitor which ispreviously charged by a direct-current source. The flash tube istriggered in response to the conduction of the main switching element. Asecond series connection of a commutating capacitor and an auxiliaryswitching element is connected in parallel with a series-connection ofthe flash tube and the main switching element, so that the auxiliaryswitching element is arranged in the same polarity as the main switchingelement. The commutating capacitor is charged in the reverse voltage tothe charged voltage of the main capacitor. The flashing current of theflash tube is terminated by rendering the auxiliary switching elementconductive in response to an ignition pulse, which is generated after anintegrated output of a detected signal from reflected light from aflashed object reaches a predetermined threshold level. In accordancewith the above construction, the object of this invention can beattained.

The second series connection may be further composed of anotherauxiliary switching element.

The principles, construction and operations of this invention will beclearly understood from the following more detailed discussion taken inconjunction with the accompanying drawings, in which the same orequivalent parts are designated by the same reference numerals, and inwhich:

FIG. 1 is a block diagram illustrating an embodiment of this invention;

FIG. 2 shows time charts explanatory of the operations of the embodimentillustrated in FIG. I;

FIG. 3 is a time-current characteristic explanatory of the operations ofthe embodiment shown in FIG. 1;

FIGS. 4, 5, 6, 7, 8,7 9, l2, l3, l4 and I5 are block diagrams eachillustrating an embodiment of this invention;

FIG. 10 shows time charts explanatory of the operations of theembodiment shown in FIG. 9; and

FIG. '11 shows waveform diagrams explanatory of the embodiment shown inFIG. 9.

With reference to FIG. I, an embodiment of this invention comprises adirect-current source I; a main capacitor 2 previously charged by thedirect-current source 1; a first series-connection of a coil 3, a flashtube 4 and a main switching element 5 which connection is connected inparallel with the main capacitor 2; a trigger circuit (6) for triggeringthe flash tube 4 in response to the conduction of the main switchingelement 5; a firing circuit 7 for rendering the main switching clementconductive; and commutation means for commutating the main switchingelement 5 and comprising a second series-connection of a commutatingcapacitor 11 and an auxiliary switching element 12 which connection isconnected in parallel with a seriesconnection of the flash tube 4 andthe main switching element. The direct-current source 1 comprises abattery 1-4, an oscillator 1-5 excited by the battery for generating anac voltage, an power transformer having a primary winding connected tothe oscillator for receiving the ac voltage and a secondary winding I-7,and diodes l-ll and l-l2. The secondary winding l-7 has terminals l-8and I-9 and an intermediate terminal l-10. Respective one terminals ofthe diodes L1] and l-l2 are connected to the terminals [-8 and I-9.Terminals l-l, l-2 and 'l-3 of the dc source are provided at theintermediate terminal l-l0, and respective other terminals of the diodesl-l l and [-12 respectively. The tenninals l-l and l-2 are connectedthrough a resistor 14 to the main capacitor 2. The terminals l-] and l-3are connected through a resistor 13 to the commutating capacitor 11. Aphotosensitve element 10, such as a photo-transistor, detects a signalfrom reflected light 9 which is reflected from an flashed object inresponse to flash light of the flash tube 4. The detected signal isintegrated in an integrator 8. When the integrated value reaches apredetermined threshold value, the firing circuit 7 generates anignition pulse P which is applied to the auxiliary switching element 12.The triggering circuit 6 comprises a series connection of a resistor 6-1and a capacitor 6-2, and a transformer 6-3. The'terminals of the seriesconnection of the triggering circuit 6 are connected to the terminal 1-1and the output of the coil 3. The primary winding of the transformer 6-3is connected across a junction between the resistor 6-1 and thecapacitor 6-2and a junction between the flash tube 4 and the mainswitching element 5. A diode I6 is connected in parallel with the flashtube 4 in the reverse polarity to the flowing direction of the flashingcurrent of the flash tube 4. A diode 15 is connected in parallel withthe coil 3 in the reverse polarity to the flowing direction of theflashing current.

In operation, the main capacitor 2 is charged by a dc voltage suppliedfrom the terminals 1-1 and 1-2 in a time constant C .R where values C land R are respectively a capacitance of the main capacitor 2 and aresistance of the resistor 14. The capacitor 11 is charged by a dcvoltage supplied from the terminals 1-1 and l-3 through the resistor 13.A trigger pulse P is applied to the control terminal of the mainswitching element 5 in response to switching-ON of a switch S as shownin FIG. 2 after the charged voltage of the main capacitor 2 exceeds thedischarge starting voltage of the flash tube 4. The switch S is providedin the associated camera or the flash device in the usual manner. Themain switching element 5 is rendered conductive in response to thetrigger P,, so that electric charge in the capacitor 6-2 is dischargedthrough the primary winding of the transformer 6-3 and the mainswitching element 5. Accordingly, a high pulse is generated at thesecondary winding of the transformer 6-3 and starts the flash of theflash tube 4. A flashing current of the flash tube 4 flows through apath: the main capacitor 2, the flash tube 4, the main switching element5, the coil 3 and the main capacitor 2. The flash tube 4 radiates flashlight in response to the flashing current. The flash light is projectedto a flashed object and reflected from the flashed object. The reflectedlight 9 is detected by the photo-sensitive element 10. The detectedvoltage is integrated in the integrator 8. When the integrated outputfrom the integrator 8 reaches a predetermined threshold value, anignition pulse P is applied from the firing circuit to the controlelectrode of the auxiliary switching element 12 for rendering theelement 12 conductive. An electronic charge previously charged in thecommutating capacitor 1 1 is discharged in a short time through a path:the capacitor 11, the auxiliary switching element 12, the main switchingelement 5, the flash tube 4 and the capacitor 11. Accordingly, since theanode-cathode path of the main switching element 5 is reversely biasedby the terminal voltage of the commutating capacitor 11, the mainswitching element 5 is turned OFF. At the same time, the electric chargeof the commutating capacitor 11 is discharged through a path: thecommutating capacitor 11, the auxiliary switching element 12, the coil3, the main capacitor 2, and the commutating capacitor 11, so that thecapacitors 2 and 11 are charged in the reverse polarities to each other.When the voltage of the capacitor 2 becomes equal to the voltage of thecapacitor 11, the discharge current of the commutating capacitor 11becomes zero while the auxiliary switching element 12 becomesnon-conductive. When the main switching element 5 is turned OFF asmentioned before, the flashing current of the flash tube is cut off andthe flash light is also terminated. Thereafter, the main capacitor 2 andthe commutating capacitor 11 are charged as mentioned above for the nextflash.

The diode 15 is employed for absorbing a counter electromotive force ofthe coil 3 induced at zero conditions of the flashing current and thecommutating current to avoid application of a high voltage to thecommutating capacitor 11.

As understood from the above, the embodiment shown in FIG. 1 is sodesigned that the main capacitor 2 and the commutating capacitor 11 arecharged by different dc sources (l-l, 1-2) and (l-l, 1-3), and so thatthe discharge current of the commutating capacitor 1 l is not passedthrough the flash tube 4. Accordingly. the flash light from the flashtube 4 is suitably terminated as shown in FIG. 3 at the time when anflashed object is flashed by a predetermined quantity of flash lightfrom the flash tube 4. Therefore. the predetermined threshold value canbe set over a wide controllable range. Moreover, since the chargedvoltage of the main capacitor 2 is not discharged until zero for eachflash. the power consumption of the battery 1-4 is low and economicaland successive flash actions can be also performed.

The diode 16 is employed for readily performing commutation of the mainswitching element 5.

The dc source 1 may be formed without the intermediate terminal l-10 asshown in FIG. 4, in which the respective one terminals of the rectifiers1-11 and l-I2 are commonly connected to the output terminal 1-8 of an acsource 1-5 in the reverse polarities to each other. The other terminalof the diode 1-11 and the other output terminal 1-9 of the ac source l-5are employed for charging the commutating capacitor 11, while the .otherterminal of the diode 1-12 and the terminal l-9 of the ac source l-5 areemployed for charging the main capacitor 2. In this embodiment, the coil3 is formed by two parts 3a and 312, so that the series connection ofthe flash tube 4 and the main switching element 5 is connected throughthe part 3a of the coil 3 to the main capacitor 2, while a seriesconnection of the diode 16 and the part 3b of the coil 3 is connected inparallel with the flash tube 4. One terminal of the series connection ofthe commutating capacitor 1] and the auxiliary switching element 12 isconnected to a junction between the diode 16 and the part 3b of the coil3. The part 3b of the coil 3 serves for improvement of the commutationof the main switching element 5.

With reference to FIG. 5, a firing circuit for generating a pulse (P ofthe embodiment of this invention comprises a resistor 7-11, a capacitor7-13 a resistor 7-12, a switch 7-14, and a transformer 7-15. Theresistor 7-11, the capacitor 7-13 and the resistor 7-12 form a seriesconnection connected in parallel with a series connection of the flashtube 4 and the main switching element 5. The primary winding of thetransformer 7-15 is connected, through the switch 7-14, in parallel withthe capacitor 7-13. The secondary winding of the transformer isconnected across the gate and the cathode of the main switching element5. The pulse P is obtained from the transformer 7-15 in response to theswitching ON of the switch 7-14.

With reference to FIG. 6, an embodiment of this invention furtherincludes constant voltage means connected in parallel with the mainswitching element 5 and comprising a constant voltage element 19, avotage regulating capacitor 18 and a resistor 17. The capacitor 18 ispreviously charged through the resistor 17 and the constant voltageelement by a terminal voltage of the main switching element 5. Aconstant voltage is obtained from the constant voltage element 19, suchas a zenor diode, by the discharge of the voltage regulating capacitor18 in response to the conduction of the main switching element. Theobtained constant voltage is supplied as the electric may be to pulsemeans, which includes the photo-sensitive element 10, the integrator 8and the firing circuit 7. In the embodiment shown in FIG. 6, theintegrator 8 comprises an integrating capacitor 8-1, a transistor 8-2,and a variable resistor 8-3. The forementioned predetermined thresholdvalue is determined by the adjustment of the variable resistor 8-3. Aswitching element 7-16 and a transformer 7-17 form the firing circuit 7for generating the pulse P when the output of the integrator 8 reaches apredetermined threshold value. The pulse P is applied to the gate of theauxiliary switching element 12.

With reference to FIG. 7,a series connection of a switching element 7-16and the primary winding of the transformer 7-17 may be connected inparallel with the voltage regulating capacitor 18. The switching element7-16 isrendered conductive when the adjusted output of the variableresistor 8-3 exceeds the predetermined threshold value determined inaccordance with the characteristic of the switching element 7-16. Theignition pulsel is obtained from the secondary winding of v thetransformer 7-l7 in response to the conduction of the switching element7-16 since the charged voltage of the voltage regulating capacitor 18 isdischarged through the conductive switching element 7-16 and thetransformer 7-17. The voltage regulating capacitor 18 is usually chargedat a voltage of 300 volts, which is still maintained at about a voltageof 150 volts after discharge through the above mentioned main switchingelement 5. Thismaintained voltage 15 and 150 volts is utilized-forgenerating the ignition pulse P is mentioned above. I

With reference to FIG. 8 illustrating another embodimentof thisinvention, an auxiliary commutating capacitor 22 is'connected across thegate and cathode of the main switching element 5, while the secondarywinding of thetransformer 7-15 is connected, through a diode 7-21, withthe anode-cathode path of the main switching element 5. A chargedvoltage of the capacitor 22 charged by the commutating current from themain commutating capacitor 11 is applied to a junction between the gateand the cathode of the main switching element so as to reversely biasthe same junction. This acts to sufficiently shorten the tum-OFF time ofthe main switching element 5, so that capacity of the main commutatingcapacitor 11 can be effectively reduced.

With reference to FIG. 9 illustrating another embodiment of thisinvention, another auxiliary switching element 12a is inserted betweenthe main commutating capacitor 11 and the flash tube 4, so that a seriesconnection of the auxiliary switching element 12a, the commutatingcapacitor 11 and an auxiliary switching element 12b is connected inparallel with the series connection of the flash tube 4 and the mainswitching element 5. In this embodiment, the capacitors 2 and l l arecharged by the same dc source since the commutating capacitor 11 ischarged through resistors 13a and 13b from outputs of the diode 1-12 andthe coil 3. The

switchingelements 12a and 12b are simultaneously controlled asunderstood from FIGS. 10 and 11. The operations of this embodiment canbe readily understood from the above disclosure, details are omitted.

In the aboveembodiments, the main switching element 5 is a siliconcontrolled rectifier (i.e. a threeterminal unidirectional thyrister,except FIG. 7 employing a silicon symmetrical switch (i.e. a twoterminalbidirectional thyrister). However, a Triac (i.e. a three-terminalbidirectional thyrister) and a fourlayer diode (i.e. a two-terminalunidirectional thyrister) may be employed as the main switching element5 as shown in FIGS. 13 and 14. Moreover. a silicon symmetrical switch(i.e. a two-terminal bidirectional thyrister), a Triac (i.e. athree-terminal bidirectional thyrister), a four-layer diode (i.e. atwo-terminal unidirectional thyrister) may be also employed as each ofthe auxiliary switching elements 12a and 12b as shown in FIGS. l2, l3and 14 in addition to the silicon controlled rectifier (i.e. athree-terminal unidirectional thyrister). A cold cathode discharge tubemay be employed as each of the auxiliary switching elements 12a and 12bas shown in FIG. 15, in which a silicon controlled rectifier is employedas the main switching element 5.

In the embodiment shown in FIG. 14, the four-layer diode 5 is driven bythe transformer 7-15 through a parallel connection of a capacitor 7-18and a diode 7-22, and the four-layer diode 12a is driven by thetransformer 7-17 through a parallel connection of a capacitor 7-20 and adiode 7-23, while the four-layer diode 12b is driven by the transformer7-17 through a parallel connection of a capacitor 7-21 and a diode 7-24.

What we claim is:

1. An electronic flash apparatus using a flash tube, comprising: a flashtube having a triggering electrode; a direct current source, a maincapacitor connected to the direct-current source and charged thereby; acoil; a first series circuit connected in parallel with said maincapacitor through said coil and comprising the flash tube and a normallynonconducting main switching element; a control switch; a triggercircuit connected to the triggering electrode of the flash tube fortriggering the flash tube in response to the conduction ofthe mainswitching element; a firing circuit connected to said control switch andresponsive to the state thereof for rendering the main switching elementconductive; a second series circuit connected in parallel with saidfirst series circuit and comprising a first auxiliary switching element,a commutating capacitor and a second auxiliary switching elementconnected in the order named and wherein the first and second auxiliaryswitching elements have the same polarity as the main switching element;charging means electrically coupled through the commutating capacitor tosaid directcurrent source for charging the commutating capacitor to avoltage having a polarity which is the reverse of that of the chargedvoltage of the main capacitor with respect to the forward current in thefirst and second auxiliary switching elements; light sensitive means forgenerating a detection signal responsive to reflected light which isreflected from a flashed object resulting from a light flash from theflash tube; an integrator receptive of said detection signal forintegrating same and developing a corresponding output signal; andcontrol means receptive of the output signal of said integrator andconnected to the first and second auxiliary switching elements fortriggering same when the output signal of the integrator exceeds apredetermined value.

2. An electronic flash apparatus according to claim 1, in which saidcharging means comprises a first charging resistor connected in parallelwith the series connection of the first auxiliary switching element andthe commutating capacitor, and a second charging resistor connected inparallel with the series connection of the second auxiliary switchingelement and the commutating capacitor.

3. An electronic flash apparatus according to claim 1, further includinga diode connected in parallel with said flash tube in the reversepolarity to the flowing direction of the flashing current of the flashtube.

4. An electronic flash apparatus according to claim 1, in which saidmain switching element comprises a three-terminal unidirectionalthyrister.

5. An electronic flash apparatus according to claim 1, in which atwo-terminal bidirectional thyrister is employed as the main switchingelement.

6. An electronic flash apparatus according to claim 1, in which athree-terminal bidirectional thyrister is employed as the main switchingelement.

7. An electronic flash apparatus according to claim 1, in which atwo-terminal unidirectional thyrister is employed as the main switchingelement.

8. An electronic flash apparatus according to claim 7, in which saidtwo-terminal unidirectional thyrister employed as the main switchingelement is driven by said firing circuit through a parallel connectionof a capacitor and a diode.

9. An electronic flash apparatus according to claim 1, in which saidfirst and second auxiliary switching elements each comprise athree-terminal unidirectional thyrister.

10. An electronic flash apparatus according to claim 1, in which atwo-terminal bidirectional thyrister is employed as each of the firstand second auxiliary switching elements.

11. An electronic flash apparatus according to claim 1, in which athree-terminal bidirectional thyrister is employed as each of the firstand second auxiliary switching elements.

12. An electronic flash apparatus according to claim 1, in which atwo-terminal unidirectional thyrister is employed as each of the firstand second auxiliary switching elements.

13. A light flashing apparatus according to claim 1, in which a coldcathode discharge tube is employed as each of the first and secondauxiliary switching elcments.

1. An electronic flash apparatus using a flash tube, comprising: a flashtube having a triggering electrode; a direct current source, a maincapacitor connected to the direct-current source and charged thereby; acoil; a first series circuit connected in parallel with said maincapacitor through said coil and comprising the flash tube and a normallynonconducting main switching element; a control switch; a triggercircuit connected to the triggering electrode of the flash tube fortriggering the flash tube in response to the conduction of the mainswitching element; a firing circuit connected to said control switch andresponsive to the state thereof for rendering the main switching elementconductive; a second series circuit connected in parallel with saidfirst series circuit and comprising a first auxiliary switching element,a commutating capacitor and a second auxiliary switching elementconnected in the order named and wherein the first and second auxiliaryswitching elements have the same polarity as the main switching element;charging means electrically coupled through the commutating capacitor tosaid direct-current source for charging the commutating capacitor to avoltage having a polarity which is the reverse of that of the chargedvoltage of the main capacitor with respect to the forward current in thefirst and second auxiliary switching elements; light sensitive means forgenerating a detection signal responsive to reflected light which isreflected from a flashed object resulting from a light flash from theflash tube; an integrator receptive of said detection signal forintegrating same and developing a corresponding output signal; andcontrol means receptive of the output signal of said integrator andconnected to the first and second auxiliary switching elements fortriggering same when the output signal of the integrator exceeds apredetermined value.
 2. An electronic flash apparatus according to claim1, in which said charging means comprises a first charging resistorconnected in parallel with the series connection of the first auxiliaryswitching element and the commutating capacitor, and a second chargingresistor connected in parallel with the series connection of the secondauxiliary switching element and the commutating capacitor.
 3. Anelectronic flash apparatus according to claim 1, further including adiode connected in parallel with said flash tube in the reverse polarityto the flowing direction of the flashing current of the flash tube. 4.An electronic flash apparatus according to claim 1, in which said mainswitching element comprises a three-terminal unidirectional thyrister.5. An electronic flash apparatus according to claim 1, in which atwo-terminal bidirectional thyrister is employed as the main switchingelement.
 6. An electronic flash apparatus according to claim 1, in whicha three-terminal bidirectional thyrister is employed as the mainswitching element.
 7. An electronic flash apparatus according to claim1, in which a two-terminal unidirectional thyrister is employed as themain switching element.
 8. An electronic flash apparatus according toclaim 7, in which said two-terminal unidirectional thyrister Employed asthe main switching element is driven by said firing circuit through aparallel connection of a capacitor and a diode.
 9. An electronic flashapparatus according to claim 1, in which said first and second auxiliaryswitching elements each comprise a three-terminal unidirectionalthyrister.
 10. An electronic flash apparatus according to claim 1, inwhich a two-terminal bidirectional thyrister is employed as each of thefirst and second auxiliary switching elements.
 11. An electronic flashapparatus according to claim 1, in which a three-terminal bidirectionalthyrister is employed as each of the first and second auxiliaryswitching elements.
 12. An electronic flash apparatus according to claim1, in which a two-terminal unidirectional thyrister is employed as eachof the first and second auxiliary switching elements.
 13. A lightflashing apparatus according to claim 1, in which a cold cathodedischarge tube is employed as each of the first and second auxiliaryswitching elements.